One thing that was mentioned here by the anti-knitting in class side and never acknowledged is that one's knitting affects more than just the professor. Would you be willing to stop if it made somebody sitting next to you or even a couple of rows down uncomfortable or distracted? Even wood needles make some clicky noise. If somebody asked you to stop, or even just looked at you uncomfortably a bunch of times, would you be considerate? There's anywhere between 20 and 500 kids in a classroom depending on the school you go to and which class. Which ones are you making the class harder for?
As a recent college and law student, I completely agree with this. I remember the woman in law school who sat one row below me and watched DVDs (with the sound off) on her laptop during class. I can't begin to describe how distracting seeing a movie playing on her screen was for me! But I felt uncomfortable asking her to stop. Knitting would be the same way. Some people will
be distracted by your knitting and yet may not be comfortable asking you to stop. While you may not appreciate the class or feel like you need to put a lot of effort into taking notes, your distracted classmates may need to give every ounce of their concentration to the professor's lectures in order to do well or even pass.
Just consider that a class that is completely unimportant to you may be vitally important to someone else, and you may be interfering with that.
And I would say the same thing to newspaper reading, loud eating, and other distracting activities that other people have mentioned. I'd love to knit all the time, but it just isn't possible or polite.